Electroplating with zinc

A great number of measurements have been reported for articles electroplated with zinc. The various aims have been evaluation of the corrosion rate of zinc that had been plated in a number of commercial cyanide-free zinc baths," comparison of the corrosion rate of a composite material (zinc with codeposits of various oxides) and of pure zinc deposits," corrosion testing of various alloyed zinc platings (Zn-Ni, Zn-Co, Zn-Fe), with or without subsequent post-treatment. Most of the work in the last category was only recorded in internal reports. The published work consists of an examination of the corrosion behavior of a ctoomated Zn-Fe... 

Produced by continuously flash electroplating with zinc— 30 to 60 g/m total on both sides. It is used when minimal corrosion resistance is required. 

At first sight, the answer would seem to be to increase the thickness of the zinc layer. This is not easily done, however, because the hot dipping process used for galvanising is not sufficiently adjustable and electroplating the zinc onto the steel sheet increases the production cost considerably. Painting the sheet (for example, with a bituminous paint) helps to reduce the loss of zinc considerably, but at the same time should vastly decrease the area available for the cathodic protection of the steel and if a scratch penetrates both the paint and the zinc, the exposed steel may corrode through much more quickly than before. 

Electroplating is achieved by passing an electric current through a solution containing dissolved metal ions as well as the metal object to be plated. The metal object acts as a cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and nonferrous metal objects are typically electroplated with aluminum, brass, bronze, cadmium, chromium, copper, iron, lead, nickel, tin, and zinc, as well as precious metals such as gold, platinum, and silver. Common electroplating bath solutions are listed in Table 7-1. 

Galvanize — (1) To stimulate living cells by means of an electric current. (2) To electroplate a metal surface with a dissimilar metal in particular, to electroplate iron with zinc. In the latter case, the term is also commonly misapplied to iron that has been dip-coated with zinc. (3) Figuratively, to restore something to life from a dormant state. 

Pennies in the United States consist of a zinc core that is electroplated with a thin coating of copper. Zinc dissolves in hydrochloric acid, but copper does not. If a small scratch is made on the surface of a penny, it is possible to dissolve away the zinc core, leaving only the thin shell of copper. Identify which element is oxidized and which is reduced in the reaction for the dissolving of the zinc by the acid. 

The final solution for corrosion protection is the coating of the surface with a film of an inert substance. This can be a layer of a metal with better protection properties than the bulk material. One example is the covering of iron or unalloyed steel by a zinc layer with a thickness of some micrometers. Zinc coating is possible by dipping into a hot zinc bath or by electroplating. The zinc layer must be further treated for sufficient corrosion protection. This can be chromatizing as described in the previous chapter or by a newly developed process free of chromate. 

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